1. Field of the Invention
The present invention relates to an optical wavelength converting apparatus and an optical wavelength converting method that carries out wavelength conversion by subjecting two beams from different light sources such as different semiconductor lasers (Laser Diodes: LDs) to sum frequency generation. More specifically, the present invention relates to an optical wavelength converting apparatus which can be utilized as a light source for laser display, optical recording, or optical measurement, which can be driven for high-speed modulation, and which can emit green laser light.
2. Related Background Art
Many attempts have been made to use nonlinear optical material to convert LD light so as to have a different wavelength. This technique provides a light source that generates laser light with a wavelength region with which no LDs have been successfully put to practical use, for example, a green or ultraviolet region. This light source is expected to be used for laser display or optical recording.
A combination of a diode pumped solid state laser (DPSS laser) and nonlinear optical material is a known method of generating green laser light using nonlinear optical material. With this method, a laser crystal such as Nd:YAG is pumped using LD light of wavelength 808 nm to oscillate laser light of wavelength 1,064 nm (DPSS laser). Then, nonlinear optical material such as KTP (KTiOPO4) is irradiated with this light to generate a second-harmonic to emit laser light of wavelength 532 nm.
It is contemplated that nonlinear optical material may be directly irradiated with LD light of wavelength about 1,060 nm to generate a second-harmonic (530 nm). In this case, output light can be modulated by directly modulating the LD. As a result, high-speed modulation can be accomplished.
On the other hand, with a so-called sum frequency generating method, two LD beams of wavelengths λ1, and λ2 are allowed to be incident on nonlinear optical material to generate light with a wavelength λ3 having a frequency equal to the sum of the two frequencies. This method is described in Japanese Patent Application Laid-Open No. 6-175180, for example. FIG. 15 shows an optical system used for this method. In the figures, reference numerals 401, 402, and 417 denote a modulation signal, a signaling LD, and an pumping LD, respectively. Two LD beams are incident, via an optical fiber 403 and a dichroic mirror 418, on nonlinear optical element 414 provided with a periodically poled layer 420. The two LD beams are converted into sum frequency light by the nonlinear optical element 414, and this light is emitted. Further, reference numerals 412, 413, 415, and 416 denote an optical parametric sum frequency converting device, a light receiving device, and a LiTaO3 substrate, and an optical waveguide respectively. Reference numerals 419, 421, and 422 denote a lens, emitting section of the optical waveguide (from which sum frequency light P3 is output), and a filter (that absorbs pumping light P2 that has not been converted) respectively. Reference character P1 denotes signal light.
However, with this type of method using an DPSS laser, the fluorescence lifetime of YAG is about 0.23 ms. Accordingly, modulation is limited to several kHz, and this method is unsuitable for high-speed modulation. Consequently, an external modulator is required for modulation, thus limiting the reduction of size of the apparatus and increasing power consumption.
Further, with the method of generating a second-harmonic using LD light of wavelength 1,060 nm, the efficiency of conversion into a second-harmonic is sensitive to the wavelength of LD light and also depends on input power. Accordingly, the LD light source must provide high power and high wavelength stability (1 nm or less). Furthermore, to modulate light so as to obtain output light with high power, a modulation current with a large amplitude must be injected into the LD light source. This requires a laser driver compatible with a large current, thus increasing costs.
Moreover, also with the sum frequency generating method, the efficiency of conversion into sum frequency light is sensitive to the wavelength of LD light. Consequently, the two LD light sources must provide high wavelength stability (1 nm or less). Furthermore, the exciting LD must provide high power.
Accordingly, with the second-harmonic generating method or the sum frequency generating method, to allow the LD light source to provide high power and high wavelength stability, it is necessary to have a DFB (Distributed Feedback), DBR (Distributed Bragg Reflector), or other laser which is grated and which has a special device configuration that enables the injection of a large current. This increases laser production costs.
Further, in general, with the second-harmonic generating method or the sum frequency generating method, the conversion efficiency is sensitive to the temperature of the nonlinear optical material. Thus, a variation in the temperature of the nonlinear optical material must be limited to several ° C. or less. This requires special means such as a temperature control device, thus limiting the reduction of size of the apparatus and increasing costs and power consumption.